In the wrought form, commercially pure aluminum is
known as 1100. It has a high degree of resistance to
corrosion and is easily formed into intricate shapes.
It is relatively low in strength and does not have the
properties required for structural aircraft parts. High
strengths are generally obtained by the process of
alloying. The resulting alloys are less easily formed
and, with some exceptions, have lower resistance to
corrosion than 1100 aluminum.

Alloying is not the only method of increasing the
strength of aluminum. Like other materials, aluminum
becomes stronger and harder as it is rolled, formed, or
otherwise cold worked. Since the hardness depends
on the amount of cold working done, 1100 and some
wrought aluminum alloys are available in several strain
hardened tempers. The soft or annealed condition is
designated O. If the material is strain hardened, it is
said to be in the H condition.

The most widely used alloys in aircraft construction are
hardened by heat treatment rather than by cold work.
These alloys are designated by a somewhat different
set of symbols: T4 and W indicate solution heat treated
and quenched but not aged, and T6 indicates an alloy
in the heat treated hardened condition.

W  Solution heat treated, unstable temper

T  Treated to produce stable tempers other
than F, O, or H

T2  Annealed (cast products only)

T3  Solution heat treated and then cold
worked

T4  Solution heat treated

T5  Artificially aged only

T6  Solution heat treated and then artificially
aged

T7  Solution heat treated and then stabilized

T8  Solution heat treated, cold worked, and
then artificially aged

T9  Solution heat treated, artificially aged, and
then cold worked

T10  Artificially aged and then cold worked

Additional digits may be added to T1 through T10 to
indicate a variation in treatment which significantly
alters the characteristics of the product.

Aluminum alloy sheets are marked with the specification
number on approximately every square foot of
material. If for any reason this identification is not
on the material, it is possible to separate the heattreatable
alloys from the non-heat-treatable alloys by
immersing a sample of the material in a 10 percent
solution of caustic soda (sodium hydroxide). The
heat-treatable alloys will turn black due to the copper
content, whereas the others will remain bright. In the
case of clad material, the surface will remain bright,
but there will be a dark area in the middle when viewed
from the edge.